Add Duplicate Character Counter in JavaScript

archive/j/javascript/duplicate-character-counter.js

@@ -0,0 +1,40 @@
+"use strict";
+
+const USAGE = "Usage: please provide a string";
+
+const isAlphaNum = (c) => /^[a-z0-9]$/i.test(c);
+
+const countDuplicates = (str) => {
+  const counts = new Map();
+
+  for (const ch of str) {
+    if (isAlphaNum(ch)) {
+      counts.set(ch, (counts.get(ch) ?? 0) + 1);
+    }
+  }
+
+  const duplicates = [];
+
+  for (const [ch, count] of counts) {
+    if (count > 1) {
+      duplicates.push(`${ch}: ${count}`);
+    }
+  }
+
+  return duplicates.length > 0
+    ? duplicates.join("\n")
+    : "No duplicate characters";
+};
+
+const run = () => {
+  const [, , input] = process.argv;
+
+  if (!input) {
+    console.error(USAGE);
+    process.exit(1);
+  }
+
+  console.log(countDuplicates(input));
+};
+
+run();